1. Field of the Invention
This invention relates to error detection in digital transmission wherein correlative level coding is employed, and more particularly relates to error detection for a modified duobinary signal.
2. Background of the Invention
Duobinary systems are explained, for example, in an article appearing in IEEE Transactions on Communications and Electronics, vol. 82, May 1963, pp. 214-218, as well as a variety of generally available publications such as IEEE Spectrum article February 1966. In particular, the duobinary system is disclosed and claimed in U.S. Pat. No. 3,238,299 entitled, "High-Speed Data Transmission System" by the present inventor. Subsequent publications by this inventor have disclosed a number of variations in the basic duobinary concepts. The duobinary technique, as originally developed, contemplated a baseband three-level, correlative level-coded signal in which the frequency spectrum of the wave increased with decreasing frequency, reaching a maximum value at a frequency which was essentially zero. Important among the additional publications of interest is the article entitled, "Correlative Digital Communication Techniques," appearing in IEEE Transactions on Communications Technology, vol. 13, June 1965, pp. 203-208. In addition, a number of correlative level-coded techniques are described in U.S. Pat. No. 3,388,330 entitled, "Partial Response Multilevel Data System." More particularly, a technique for converting a binary signal into a modified duobinary signal and the particularities of the characteristics of the modified duobinary signal are described in considerable detail in U.S. Pat. No. 3,457,510 entitled, "Modified Duobinary Data Transmission" by the present inventor. The subject U.S. Pat. No. 3,457,510 is incorporated herein by reference. A digital technique for generating a modified duobinary signal is contained in the copending application, Ser. No. 623,292, filed Oct. 17, 1975, and the present inventor is a coinventor of that application.
A technique for the detection of errors in a modified duobinary signal is disclosed in U.S. Pat. No. 3,461,426. In the subject error detection patent, it is to be noted that the transmitted modified duobinary waveform and the binary waveform reconstituted therefrom by decoding are monitored and coincidences detected between extreme level signals of the former and correlated portions of the latter to indicate errors. In contrast, the error detector of the present invention does not require the use of the binary output signal obtained by decoding in the receiving circuitry. Further, the prior error detection technique does not teach the concept of dividing the modified duobinary into two (odd and even) pulse trains each of which is representative of a bipolar pulse train.